1. Field of the Invention
The present invention relates to a charge/discharge control circuit which is capable of controlling the charge/discharge of a secondary battery by the on/off operation of a switching circuit and a charging type power supply circuit using the circuit.
2. Description of the Related Art
As a conventional charging type power supply device formed of a secondary battery, there has been known a power supply device shown by a circuit block diagram of FIG. 2. This structure is disclosed in, for example, xe2x80x9ccharging type power supply devicexe2x80x9d of Japanese Patent Application Laid-Open No. Hei 4-75430. That is, a secondary battery 101 is connected to an external terminal xe2x88x92V0105 or +V0104 through a switching circuit 103. A charge/discharge control circuit 102 is also connected in parallel with the secondary battery 101.
The charge/discharge control circuit 102 has a function of detecting a voltage across a secondary battery 101. In the case where the secondary battery 101 is in an overcharged state (a state where the battery voltage is higher than a given voltage value; hereinafter referred to as xe2x80x9covercharge protecting statexe2x80x9d) or in an overdischarged state (a state where the battery voltage is lower than the given voltage value; hereinafter referred to as xe2x80x9coverdischarge protecting statexe2x80x9d), a signal is outputted from the charge/discharge control circuit 102 so that the switching circuit 103 turns off. Also, if discharging operation stops when the external terminal +V0104 reaches a certain voltage, it is possible to limit a current that flows in the switching circuit 103. That is, the discharging operation can stop (over-current control) when an excessive current flows in the switching circuit 103. Hereinafter, this state is referred to as xe2x80x9cover-current protecting statexe2x80x9d.
As another example of the conventional charging type power supply device formed of a secondary battery, there has been also known a power supply device shown by a circuit block diagram of FIG. 3. This circuit is designed such that the switching circuit 103 shown in FIG. 2 is connected in series to a negative pole 111 of the secondary battery.
FIG. 4 shows a conventional example of a circuit block diagram of a specific charge/discharge control circuit. A secondary battery 101 is connected to an external terminal xe2x88x92V0105 through a switching circuit 103. The switching circuit 103 is made up of two n-channel FETS. A voltage across the secondary battery 101 is detected by a charge/discharge control circuit 102. The charge/discharge control circuit 102 is made up of an overcharge detection comparator 119, an overdischarge detection comparator 118, an over-current detection comparator 117, a reference voltage circuit A 116, a reference voltage circuit B 114, a voltage divider circuit A 120, a voltage divider circuit B 121, an output logic control circuit 124, etc. The charge/discharge control circuit 102 is connected to the switching circuit 103 by signal lines 107A and 107B to send out an on/off signal of the switching circuit 1039 A charger 108 for charging the secondary battery 101 and a device (a load viewed from the secondary battery) drivable by the secondary battery 101 are connected between the external terminal +V0104 and xe2x88x92V0105. An FET-A 112 and an FET-B 113 are connected in series to the external terminal xe2x88x92V0105 or +V0104.
The overcharge detection comparator 119 and the overdischarge detection comparator 118 have a function of comparing the voltage across the secondary battery 101 with the voltage across the reference voltage circuit A 116. Since the output logic control circuit 124 sends out a signal to the terminals 125A and 1258 in accordance with the outputs of the respective comparators 119 and 118, the gate voltages of the respective FETs vary in accordance with the respective states so as to turn on/off the charging and discharging operation with respect to the secondary battery. For example, in the overcharge state, the plus input terminal voltage of the overcharge detection comparator 119 becomes higher than the reference voltage A 116, and the output of the comparator 119 is inverted from low to high. When the output signal is inputted to the output logic control circuit 124, the gate voltage of the FET-B 113 in the switching circuit changes from high to low. As a result, the discharge current does not flow in the secondary battery 101, to thereby stop the charging operation.
The over-current detection comparator 117 compares the external terminal xe2x88x92V0105 with the voltage across the reference voltage circuit B 114 and outputs a signal in accordance with respective states to the output logic control circuit 124. In the over-current protecting state, the output logic control circuit 124 sends out a signal to the FET-A 112 so as to stop the discharging operation while sending out a signal to the FET-C 126s, to thereby pull down the external terminal xe2x88x92V0105 by a resistor 127. when the load 109 is out of the over-current protecting state after the detection of the over-current, the pull down becomes necessary to stabilize the external terminal xe2x88x92V0105 to the voltage of the reference voltage B 114 or lower and return the state to a normal state.
Although it is possible to realize a switch using one FET instead of the above switching circuit, it is necessary to change the gate potential of the FET as well as the substrate potential in order to achieve the above. If the above change is not made, because the source potential of the FET can be made higher than the drain potential, the charge/discharge control described above is impossible. At present, it is general to control the charge/discharge by two FETS.
However, in the charge/discharge control circuit thus structured, when the load is out of the over-current protecting state and the charge/discharge control circuit returns to the normal state, there arise the following drawbacks which will be described with reference to FIG. 4.
In order that the charge/discharge control circuit is automatically restored when the load 109 is out of the over-current protecting state, it is necessary that the external terminal xe2x88x92V0105 becomes a voltage Va of the reference voltage B 114 or lower. To achieve this, an open-circuit impedance when the load is out of the over-current protecting state must be a constant value (Ra) or more with respect to the resistor 127 (Rb). If the secondary battery 101 is Vb, Ra can be represented by the following expression.
Ra greater than Rb(Vb/Vaxe2x88x921)xe2x80x83xe2x80x83(1)
There is known that Vb is generally set to about 3.5 V during the normal operation. In the case where the charge/discharge control device is formed of a semiconductor integrated circuit, since the potential of the external terminal xe2x88x92V0105 becomes lower than the potential of the negative pole 111 of the secondary battery when the charger 108 is connected to the charge/discharge control device, and a current flows through the resistor 127 (Rb) from the drain of the FET-C 126, Rb is generally set to about 100 Kxcexa9 in order to limit that current. Va is generally set to about 0.1 V which is set on the basis of the on resistances of the FET-A 112 and the FET-B 113 and currents flowing therein.
If the above-mentioned values are inputted to the expression (1), Ra is represented as follows:
Ra greater than 100 Kxcexa9(3.5 V/0.1xe2x88x921)=3.4 Mxcexa9
There is the possibility that the actual open-circuit impedance becomes 1 Mxcexa9 or less according to the characteristics of such as the mounted substrate, and the charge/discharge control circuit may not return to the normal state merely when the load is out of the over-current protecting state.
Therefore, an object of the present invention is to solve the above problem.
In order to achieve the above object, according to the present invention, there is provided a charge/discharge control circuit having another return detection voltage different from a detection voltage for detecting an over-current when the charging type power supply device returns to a normal state from an over-current state.